Hydrogen bonds in hair are broken by which of the following methods?

Hydrogen bonds in hair are specifically affected by heat and water because these bonds are relatively weak and can be easily altered. When hair is exposed to water, the moisture disrupts these bonds, allowing the hair's structure to change temporarily. For instance, when hair is wet, it becomes more pliable and can be styled or molded more easily because the hydrogen bonds are broken. Heat also plays a significant role; applying heat through styling tools can break hydrogen bonds in a similar manner, allowing the hair to form new shapes or styles as the bonds re-form upon cooling.

In contrast, chemical products primarily work on disulfide bonds or alter the hair's pH and can lead to longer-lasting changes, but they do not specifically target the breaking of hydrogen bonds. Cold temperatures do not effectively disrupt hydrogen bonds and instead can potentially strengthen them by reducing molecular movement. Air exposure generally does not cause breaking of these bonds either, as it does not provide the necessary moisture or heat to influence their stability significantly.